Why Is the Moon’s Orbit Tilted?
published during a waxing gibbous moon.
11/11/2016

moon-pass

Credit: NASA/NOAA

By now you may have heard that the moon maybe isn’t just a hunk of rock, or that its dark side actually gets some sun once in a while. And it’s OK that we believed otherwise: Myths are like theories gone a little haywire because they just sound cooler (and make for better Pink Floyd albums). The moon had been surrounded with a mystique for eons before humans stuck things in it and supercomputers were able to crunch complex scenarios about its birth and evolution. There’s one itch that astronomers hadn’t been able to get even the smartest machines to scratch, though: why the moon’s orbit is tilted.

The moon goes around the Earth with an inclination of about 5 degrees. In other words, it’s tilted, meaning its orbit doesn’t perfectly match up with our equator. Even more bananas than that, the moon’s rotations have even more quirks, which leads to graphs like the one below that makes this author’s eyes glaze over faster than a fresh Krispy Kreme.

orbit tilted

Credit: Public Domain / Creative Commons

To keep your brain’s wires from tangling, let’s just focus on that little red arrow that points to the inclination, and the number 5.

In a study published on Halloween, researchers revealed how the moon got its tilt: an extreme impact that literally vaporized early Earth. The dominant theory of how the moon even came to exist is also a high-impact scenario, so it doesn’t veer too far from the long-running hypothesis.

In short, that theory points to pure chaos. Theia, a planetary-mass object roughly the size of Mars, is thought to have come hurtling at Earth and crashed into it head-on. The moon eventually formed from the debris. Till now, though, the tilt didn’t fit into the narrative, because the sun, which has the most mass in the solar system, dictates other major planets’ orbits via its equator. But why wouldn’t the moon play nice with Earth in a neat, even-keeled orbit amid the gravitating rubble?

One of the reasons that the Theia theory is so compelling is because the same isotopes were found on Earth and the moon. Isotopes are elements that have varied numbers of neutrons. Take Earth’s oxygen, which is O-16, for example. Earth also has trace amounts of O-17 and O-18. While other cosmic bodies’ isotopes vary based on where they formed, the moon and Earth’s were found to be the same, which astronomers say points to the moon coalescing from the Theia collision. The newer models of the theory all check out, including one where the impact was so high that not only did Theia vaporize, but Earth did, too, all the way down to its rocky skivvies.

That is, until that pesky orbit pops its head back up like an unrelenting mole. After a big impact, the moon would form from a disk around the moon’s equator, senior study author Sarah Stewart told Space.com. That means it would be expected to stay near the equator. This time, though, scientists think they have the mole whacked.